Why doesn't a long lasting vaccine against common cold exist yet?
Viruses that present a high mutation rate like the virus that causes the common cold escape easily from the action of vaccines against them. After a primary immune response (natural or artificially induced) against the virus in the next season of infection new mutant resistant strains appear and the protection obtained with the immune response of the last season is lost. (One could say that the high mutation rate is a form of “immunization” found by these viruses.)
